It's possible, question is more to they do it.
Users of a Content Owner on YouTube (MCNs/ large brands/ Content ID users) that reach the minimums (I believe it's around 50m-100m views per month) can apply for the custom/ partner sales program, once they complete some certification.
This allows the above to sell/ serve their own advertising in CPV (overview, trueview & standard view) and CPM (display) formats via an external ad server or via YouTube/ Doubleclick ad servers.
The only benefit to an external ad-server is being able to serve one campaign on multiple sites if they are say a large scale network using sites like DailyMotion aswell.
But it's not easy, the minimum budget per campaign is $10,000-$25,000, there is alot of paperwork and you need technology capable of generating the XML files.
Average setup time per campaign for a team of 1 sales agent & 2 account managers would be around 2-3 weeks, pre-planning would be around 1-2 weeks before that, and after the 2-4 week campaign there would be another 1-2 week accounting & statistics period.
Many large networks such as Machinima have transitioned away from the instream advertising model because it is very manual-effort focused, teams need to be atleast 3-4 people and cannot really be scaled.
Besides the fact that branded content/ sponsorships tend to be cheaper, easier to setup and scaled easier.
I'd say of the top 50 networks, you'd be lucky if 25 are even approved for sales and if 10 do it regularly.
And of that, most campaigns would be run on no more than 20-50 channels & would not last longer than a few weeks.
Those I know of, Fullscreen & Maker both have 20+ people on sales but again really most large channels will not even see much/any let alone smaller ones.